Every day, the NSA collects almost five billion records tracing the movements of hundreds of millions of cell
phones from around the world, according to report in the Washington Post based on an Edward Snowden-provided classified document.

A giant database stores the
information, which the NSA gathers by tapping into cables that
connect cell phone networks in the U.S. and internationally.
While the NSA does not target U.S. citizens, information about
millions of Americans' using cell phones abroad is collected
"incidentally."

"[T]here is no element of the
intelligence community that under any authority is intentionally
collecting bulk cellphone location information about cellphones in
the United States," Robert Litt, general counsel for the Office of
the Director of National Intelligence, told the Post.

The programs used by the NSA, collectively called
CO-TRAVELER, analyzes the bulk data—the vast majority of which is
taken from cellphones owned by people of no national security
interest to the NSA— in order to discover the associates of
established intelligence targets.

Still, location data, especially when
aggregated over time, is widely regarded among privacy advocates as
uniquely sensitive. Sophisticated mathematical techniques enable NSA
analysts to map cellphone owners' relationships by correlating
their patterns of movement over time with thousands or millions of
other phone users who cross their paths. Cellphones broadcast their
locations even when they are not being used to place a call or send a
text.

CO-TRAVELER and related tools require
the methodical collection and storage of location data on what
amounts to a planetary scale. The government is tracking people from
afar into confidential business meetings or personal visits to
medical facilities, hotel rooms, private homes and other
traditionally protected spaces.

"One of the key components of
location data, and why it's so sensitive, is that the laws of
physics don't let you keep it private," Chris Soghoian, the
principal technologist at the American Civil Liberties Union, told
the Post.

Even using disposable cellphones or
turning off your phone between calls triggers CO-TRAVELER, which can
track when a new cellphone is activated immediately or shortly after
another is disconnected.

"[T]he only way to hide your location
is to disconnect from our modern communication system and live in a
cave," Soghoian said.